Light emitting diodes (“LEDs”) are well known solid state lighting devices that are capable of generating light. LEDs generally include a plurality of semiconductor layers that may be epitaxially grown on a semiconductor or non-semiconductor substrate such as, for example, sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrates. One or more semiconductor active layers are formed in these epitaxial layers. When a sufficient voltage is applied across the active layer, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the active layer. As the electrons and holes flow toward each other, some of the electrons will “collide” with a hole and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
LEDs typically have a narrow wavelength distribution that is tightly centered about a “peak” wavelength (i.e., the single wavelength where the radiometric emission spectrum of the LED reaches its maximum as detected by a photo-detector). For example, the spectral power distributions of a typical LED may have a full width of, for example, about 10-30 nm, where the width is measured at half the maximum illumination (referred to as the full width half maximum or “FWHM” width). Accordingly, LEDs are often identified by their “peak” wavelength or, alternatively, by their “dominant” wavelength. The dominant wavelength of an LED is the wavelength of monochromatic light that has the same apparent color as the light emitted by the LED as perceived by the human eye. Thus, the dominant wavelength differs from the peak wavelength in that the dominant wavelength takes into account the sensitivity of the human eye to different wavelengths of light.
As most LEDs are almost monochromatic light sources that appear to emit light having a single color, LED lamps that include multiple LEDs that emit light of different colors have been used in order to provide solid state light emitting devices that generate white light. In these devices, the different colors of light emitted by the individual LED chips combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the source red, green and blue LEDs.
White light may also be produced by surrounding a single-color LED with a luminescent material that converts some of the light emitted by the LED to light of other colors. The combination of the light emitted by the single-color LED that passes through the wavelength conversion material along with the light of different colors that is emitted by the wavelength conversion material may produce a white or near-white light. For example, a single blue-emitting LED chip (e.g., made of indium gallium nitride and/or gallium nitride) may be used in combination with a yellow phosphor, polymer or dye. Blue LEDs made from indium gallium nitride exhibit high efficiency (e.g., external quantum efficiency as high as 60%). In a blue LED/yellow phosphor lamp, the blue LED chip produces an emission with a dominant wavelength of about 445-470 nanometers, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nanometers in response to the blue emission. Some of the blue light passes through the phosphor (and/or between the phosphor particles) without being down-converted, while a substantial portion of the light is absorbed by the phosphor, which becomes excited and emits yellow light (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light may appear white to an observer. Such light is typically perceived as being cool white in color. In another approach, light from a violet or ultraviolet emitting LED may be converted to white light by surrounding the LED with multicolor phosphors or dyes. In either case, red-emitting phosphor particles (e.g., a CaAlSiN3 (“CASN”) based phosphor) may also be added to improve the color rendering properties of the light, i.e., to make the light appear more “warm,” particularly when the single color LED emits blue or ultraviolet light.
LEDs are used in a host of applications including, for example, backlighting for liquid crystal displays, indicator lights, automotive headlights, flashlights, specialty lighting applications and even as replacements for conventional incandescent and/or fluorescent lighting in general lighting and illumination applications. In many of these applications, it may be desirable to provide a lighting source that generates light having specific properties.
LEDs are a class of luminescent materials: those that absorb energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. A luminescent material in powder form is commonly called a phosphor, while a luminescent material in the form of a transparent solid body is commonly called a scintillator. Thus a powder form of a scintillator may be referred to as a phosphor.
Most useful phosphors emit radiation in the visible portion of the spectrum in response to the absorption of the radiation which is outside the visible portion of the spectrum. Thus, the phosphor performs the function of converting electromagnetic radiation to which the human eye is not sensitive into electromagnetic radiation to which the human eye is sensitive. Most phosphors are responsive to more energetic portions of the electromagnetic spectrum than the visible portion of the spectrum. Thus, there are phosphors which are responsive to ultraviolet light (as in fluorescent lamps), electrons (as in cathode ray tubes) and x-rays (as in radiography).
The material properties of scintillators vary greatly based on the specific chemical composition of the scintillator. These properties include scintillator efficiency, primary decay time, afterglow, hysteresis, luminescent spectrum, x-ray stopping power, and resistance to radiation damage. The efficiency of a luminescent material is the percentage of the energy of the absorbed stimulating radiation which is emitted as luminescent light. When the stimulating radiation is terminated, the luminescent output from a scintillator decreases in two stages. The first of these stages is a rapid decay from the full luminescent output to a low, but normally non-zero, value at which the slope of the decay changes to a substantially slower decay rate. This low intensity, normally long decay time luminescence, is known as afterglow. Specifically, afterglow is the light intensity emitted by the scintillator at 100 milliseconds after the x-ray excitation ceases, reported as a percentage of the light emitted while the scintillator is excited by the radiation. Afterglow provides a background luminescent intensity, which is a noise contribution to the photodetector output. In some cases, afterglow is increased by the presence of impurities, and in other cases, afterglow is decreased by the presence of impurities.
Yttrium aluminum garnet (Y3Al5O12 or YAG) has long been used in many industrial applications. When doped with cerium and most often used for phosphors though also used for scintillators, the Ce undergoes a 5d to 4f transition resulting in broad band yellow light centered at 550 nm. Many metal doped YAG materials display a similar luminescent property. Recently, several studies have demonstrated significant UV defect luminescence centered around 300 nm under cathodoluminescence and ionizing radiation. This limits the efficiency of the cerium luminescence at 550 nm and hinders desired scintillation emission in the visible spectrum.
UV defect emission has been attributed in the past to one or more of several intrinsic defects; however, the prevailing thought now suggests yttrium on aluminum antisites (YAl3+) as the primary defect responsible. Unfortunately, antisites have been shown to be the most energetically favorable intrinsic defect in YAG. Further, YAl3+ on the aluminum octahedral site is the most likely intrinsic defect to be thermally produced in melt grown YAG as it is much more energetically preferable to the opposite antisite, AlY3+.
Therefore it would be desirable to develop compositions of inorganic luminescent materials with decreased UV defect emission.